Zinc-based coated steel sheets are widely used in a wide range of fields, especially for automotive body applications. In such applications, the zinc-based coated steel sheets are press-formed for use. However, the zinc-based coated steel sheets have a disadvantage that the zinc-based coated steel sheets are inferior in press formability to cold-rolled steel sheets. This is because the sliding resistance of the zinc-based coated steel sheets to press molds is higher than that of the cold-rolled steel sheets. A zinc-based coated steel sheet is hard to be fed into a press mold at a portion which has high sliding resistance to a mold or a bead; hence, the steel sheet is likely to be fractured.
In particular, in a galvanized steel sheet (hereinafter referred to as GI in some cases), a phenomenon in which sliding resistance further rises due to the adhesion of a coating to a mold (mold galling) occurs to cause cracking halfway through continuous press forming, thereby negatively affecting the productivity of automobiles.
Furthermore, from the viewpoint of tightening regulations on CO2 emissions in recent years, the usage rate of high-strength steel sheets tends to increase for the purpose of reducing automobile weight. The use of a high-strength steel sheet increases the surface pressure during press forming and therefore the adhesion of a coating to a mold is a more serious problem.
As a method for solving the above problem, Patent Literatures 1 and 2 disclose a technique for enhancing press formability in such a manner that a galvannealed steel sheet (hereinafter referred to as GA in some cases) which is alloyed is temper-rolled, is brought into contact with an acidic solution with a pH buffering action, is left for 1 second to 30 seconds after the end of contact, is water-washed, and is then dried such that zinc-based oxides are formed on a surface layer of the GA.
As for the GI, the GI has particularly low surface activity. This is because a small amount of Al is added to a galvanizing bath for the purpose of adjusting the alloying reaction of base iron with zinc, Al oxides derived from Al in the bath are present on a surface of the galvanized steel sheet, and the galvanized steel sheet has a higher concentration of Al oxide on the surface as compared to the GA.
As a method for forming the zinc-based oxides described in Patent Literatures 1 and 2, Patent Literature 3 discloses a method in which such a GI with low surface activity is brought into contact with an alkali solution before being brought in contact with an acidic solution such that surface Al oxides are removed, a surface is activated, and the formation of oxides is promoted.
As a method for forming an oxide layer containing a crystalline structure substance represented by Zn4(SO4)1-x(CO3)x(OH)6.nH2O, Patent Literature 4 discloses a method in which a similar GI with low activity is brought into contact with an alkali solution before contact with an acidic solution such that surface Al oxides are removed, a surface is activated, and the formation of oxides is promoted.
Patent Literature 5 discloses a method in which a steel sheet coated with a Zn—Al-based coating containing 20% to 95% by mass Al is brought into contact with an alkali solution and then HF is added to an acidic treatment solution such that the formation of an oxide layer is promoted.